Why God bless those who bless Ishmael's descendants also? Just like Israel?

God did not do so. Nowhere does it give the same blessing to Ishmael as He does to Isaac. This blessing is repeated to Isaac and Jacob, also. Ishmael was told he would be made into a nation but was not told that God would bless those who blessed you and curse those who cursed you. This was restricted to those people who received the birthright- Israel.